I. Field of the Invention.
The present invention relates to solar reflectors based on a tensioned polymeric reflective membrane and the invention relates specifically to the backplane and supporting periphery structure that tensions the membrane. It also pertains to the category of flat reflectors used as a part of a heliostat for directing and/or concentrating electromagnetic energy.
II. Background of the Related Art.
The purpose of solar reflectors are to direct electromagnetic/solar energy to a specified target where it can be used as radiant heat, light, or converted to another form of energy. By aiming multiple flat mirrored reflectors at a single target, a concentration of the solar energy occurs. Concentrating solar energy is advantageous because it provides greater temperatures that can be better utilized for a large variety of applications.
A heliostat is made up of one or more reflector element(s) and a mechanized solar tracking device that is used to maintain the proper positioning of the reflector(s) to bisect the angle of the Sun so the solar energy is directed to a static target or receiver as the sun crosses the sky.
Directly exposing and concentrating solar energy to a target is the most efficient form of solar energy capture. Tracking technologies such as microprocessors, GPS modules, XYZ axis sensing and positioning modules and stepper motors that are used to drive heliostats have become very cost effective, but the relative complexity and costs of the heliostat and receiving target devices have remained high. The time for a return on investment on heliostat arrays can easily be greater than 20 years and in some cases never break even because of maintenance costs. Due to the cost disadvantages, it is difficult for the solar industry to compete on price per kilowatt delivered as compared to other competing energy sources.
A primary reason a heliostat is expensive is the reflective optics must maintain their designed contour while withstanding extreme environmental conditions. Flat reflective elements are typically made of mirrored glass lamented to a structurally rigid core and attached to a heavy metal frame. All of the mechanical specifications of the heliostat have to be designed to reliably support and accurately position the heavy reflector element and at the same time withstand rain, wind, hail, dust and heat. The most significant contributing factors to the cost are the type of materials used, weight and complexity of the reflector element which increases the overall cost of the reflector element. These factors contribute to increased cost of the drive and pivot mechanisms to support the weight and provide reliable operation under extreme environmental conditions.
Jonathan Switkes (U.S. Pat. 2013/0000692 Jan. 3, 2013) laminates a rear plate, an intermediate mesh and a front reflective glass element to reduce thermal stress and improve planar flatness and structural rigidity. This is then mounted to a metal frame that provides the interface to the heliostat drive mechanism.
Using glass mirrored based heliostats presents other problems if heliostats are ever to become practical for residential and light commercial use. The problems are the liability of safety due to possible glass breakage and the expense of replacement.
Attempts to improve safety, reduce weight and cost by using polymeric membranes with reflective coating have been somewhat successful. They are used in solar trough concentrators, inflatable dish style reflectors, and sealed drum shaped collectors where a vacuum is placed inside the seal drum to produce a concave shape on the membrane to produce a concentrating dish style reflector. In most cases the weight and cost was not significantly reduced due to adding other infrastructure into the design to compensate for the severe bending and twisting forces on the frame due to the stretching of the membrane across the frame.
Mark Earle Hutchinson (U.S. Pat. No. 3,733,116 May 15, 1973) introduces a wedging technique to offset the bending and twisting of the frame when the membrane is stretched across the frame.
Eugene Martinez (U.S. Pat. No. 3,877,139 Apr. 15, 1975) created a glassless mirror using a metalized polyester mirror film attached to a formed metal pan.
Charles Kojabashian (U.S. Pat. No. 3,880,500 Apr. 29, 1975) describes a planar mirror improvement by stretching reflective metalized film membrane on both sides of a frame to equalize the forces that would warp and distort the frame if the film were placed only on one side of the frame.
The mentioned art forms address compensating for the undesirable forces created when stretching a membrane across a frame but do not address maintaining a constant tension over changing thermal conditions. Specifically attaching membranes to a supporting structure present a big problem that needs to be compensated for when exposed to changing thermal conditions. The problem is due to different expansion coefficients of the membrane relative to the different materials used in the supporting structure. In many cases it is very difficult to maintain an optically flat reflective membrane surface even with a small temperature change of only a 10 degrees centigrade. The result is a rippling effect of the membrane which grossly distorts and diffuses the reflected solar energy. The problem increases as the size of the reflective element increases. One solution includes adding a heating element around the perimeter to maintain a constant temperature for the supporting structure.
Douglas Evan Simmers (U.S. Pat. No. 2008/0137271 A1 Jun. 12, 2008) implements a circumferential heating element in a drum shaped frame to compensate for differences in thermal coefficients of the membrane and frame components.
Other applications do not attempt to maintaining an optically flat membrane because of the rippling caused by thermal change. Instead they exert a positive or negative atmospheric pressure in various configurations that creates a concave shape for concentrating the solar energy to a focal point.
Patrick Soliday (U.S. Pat. No. 5,680,262 Oct. 21, 1997) utilized a heavy frame and with multiple pneumatic cylinders to maintain tension in a stretched planar membrane as it is attached and sealed to both sides of a circular frame to form a sealed drum. A negative atmospheric pressure (a vacuum) is applied to the inside of the drum imparting a parabolic shape to both membranes. The additional negative pressure provides some compensation for thermal changes due to the constant tension on the membrane provided by the vacuum.
Safety issues also exist for concave or dished reflectors that densely concentrate the solar energy to a focal point. These present a potential fire and/or health hazard if the focal point would go astray and land on a material with a low combustible temperature or create burns on human skin. These would not be practical for residential space heating and lighting.
Each of the previously mentioned teaching do not address or provide the ability to easily replace a defective or damaged membrane. The cost of on going maintenance on such devices can easily offset any monetary gains from solar energy capture.